The prior art ultrasonic device for applying cavitation force to plaque in a human artery is described in U.S. Pat. No. 4,920,954, which is assigned to the same assignee as this application. Related prior art patents and publications are described in such prior art U.S. Pat. No. 4,920,954. Another related prior art patent is U.S. Pat. No. 4,870,953.
Ultrasound devices have become means of disintegrating and removing tissue within the human body. For example, the phacoemulsifier for removing cataracts, the ultrasonic surgical aspirator for removing tumors, the ultrasonic kidney stone breaker, the ultrasonic prosthesis remover, the ultrasonic atheroma remover for blood vessels, and the ultrasonic wire to eliminate arrhythmia.
It is highly desirable for such a device to deliver ultrasonic energy percutaneously, from outside the body, to the tissue in question by means of a long wire. So doing avoids an open field operation. However, one of the problems involved with directing sonic energy down the length of a wire is that of unwanted transverse or perpendicular motion of the wire. When the wire vibrates transversely instead of axially, the tip amplitude, or forward-and-back motion, is diminished or eliminated so that the production of cavitation, i.e., the making and breaking of bubbles, is diminished as well. Since the purpose of the ultrasonic wave guide is to establish cavitation at the distal end, the usefulness of the sonic energy transfer is reduced as the wire gets more flexible and side-to-side motion increases.
In order to reach the kidney from the urethra, for example, to break a kidney stone, or to reach the coronary arteries from the groin to disintegrate an arterial occlusion, a very thin wire is necessary since the blood vessel or duct is usually sinuous. The wire must be flexible enough so that the artery or ureter or other duct will not be injured or straightened. Also it must be thin and flexible to avoid bending stresses within the wire itself which would cause heating by absorbing the moving sound wave.
In the past, when ultrasonic wires are inserted into the body, they are generally accompanied by a surrounding catheter. The catheter has several functions: to protect the artery or duct from puncture as the wire is inserted, to cool the wire which might otherwise got red hot, to irrigate the distal sonicated tissue or stone, or to aspirate the disintegrated debris. When the ultrasonic wire is used in blood vessel work, the catheter is sometimes used to infuse heparin, saline, contrast medium, or streptokinase during the procedure for their chemical effect in the artery. The inside diameter and thickness of the catheter is not important to the proper operation of the wire and was not previously described in the patent or scientific literature. If the catheter were made smaller in diameter, it could not perform the functions for which it was designed, such as removing debris, or infusing heparin. Similarly, the plastic material of which the catheter is made was also not important in the past to the users of the ultrasonic wire.
The catheters used in the past with ultrasonic wires did not prevent transverse motion in any way. In fact, U.S. Pat. No. 4,870,953 by Robert J. Siegel, taught that it was necessary to maximize transverse motion in an ultrasonic waveguide for disruption of arterial occlusions. The only mention of the prevention of transverse motion in the patent literature is made in U.S. Pat. No. 4,920,954. Here, this motion is minimized by the type of wire used, the type of horn energizing the wire, and the electronic control feeding the system.
One problem with the prior art ultrasonic device is that as the wire is made thinner, or of a more flexible metal such as titanium or aluminium, the wire when energized by sound pulses from one end, tends to whip sideways. sinusoidally, and the wire tip can be seen to stop its axial motion. The unwanted side-to-side motion is also dangerous, since it tends to break the wire, usually at the point of connection to the sound source.